Presuming you can keep the bullets in the kill zone: ~1000 - 1100 yds with the highest BC bullets like Berger VLDs. This requires a lot of practice, an exceptional skill level, some finely tuned loads, and a top notch rifle combo. Even then I wouldn't advise those ranges for black bear unless you're deadly accurate that long. They can be difficult to recover if they aren't pasted well on the first hit.